salt                  package:Rlab                  R Documentation

_T_h_e _e_f_f_e_c_t _o_f _s_a_l_t _o_n _i_c_e _m_e_l_t_i_n_g

_D_e_s_c_r_i_p_t_i_o_n:

     When salt comes into contact with ice, it tends to break apart
     into individual ions which then interact with the frozen water and
     disrupt hydrogen bonds that have formed between ice molecules. 
     This lowers the melting temperature of ice, and it was
     hypothesized that the melting process would be hastened.  The data
     set 'salt' was collected during an experiment to determine whether
     varying the type and amount of salt applied to a specific amount
     of ice has an effect on the interval required to melt that ice.

_U_s_a_g_e:

     data(salt)

_F_o_r_m_a_t:

     A data frame with 24 observations on the following 3 variables.

     _t_y_p_e : type of salt (rock salt or table salt)

     _a_m_o_u_n_t : amount of salt used (in teaspoons)

     _t_i_m_e : time for ice to melt (in seconds)

_D_e_t_a_i_l_s:

     Background: The Effect of Salt on the Rate at Which Ice Melts

     In those sections of the country that experience winter as a time
     of snow and ice, salt is often spread on roadways in an attempt to
     counter the hazardous consequences of accumulated ice.  Ice is
     formed when the relatively disordered molecules in liquid water
     reach a temperature of 32 degrees F (0 degrees C) and begin to
     "nucleate" or form solid ice crystals consisting of ordered water
     molecules.  Salt, when in contact with ice, tends to break apart
     into individual ions (i.e. sodium and chloride) which then
     interact with the water and disrupt the hydrogen bonds that have
     formed between water molecules.  Since no covalent bonds are
     broken or formed, the resulting chemical "solvation" is not
     considered to be a chemical reaction.  However, the end result
     from the introduction of salt is that the ice crystals are
     disrupted and liquid water is achieved.

     The purpose of the current experiment is to study the effect of
     salt on the rate at which ice melts.  More specifically, the
     experiment is being conducted to answer the following questions:

     1. Does varying the amount of salt applied to a constant quantity
     of ice result in a change in the rate of melting?

     2. Does the type of salt used have an effect on the melting rate?

     The first question is of interest as it relates to issues such as
     the cost of salt and the potential harmful effects of its use on
     pavement. If increasing the amount of salt applied to a given
     quantity of ice is not accompanied by an increase in melting rate,
     any application of salt beyond minimal amounts would constitute a
     waste of public money and possibly cause unnecessary damage to
     public roadways.  It is hypothesized that the relationship between
     amount of salt used and the time required to completely melt a
     given quantity of ice is negative and significant.

     Likewise, the second question seeks to address the possibility
     that dissimilar forms of salt may produce different rates of
     melting.  To answer this question, table salt and rock salt were
     included in the experimental design.  Although both are chemically
     similar, rock salt consists of larger crystals than does the
     typical table salt bought in local supermarkets.  Given the
     greater density and more efficient packing of NaCl molecules
     within the larger rock salt crystals, a specified volume of rock
     salt will likely contain a greater number of salt molecules than a
     similar volume of the less tightly packed table salt crystals. 
     Therefor, it is hypothesized that rock salt will result in faster
     melting times than table salt.

     Materials

     Tap water 42 - 6 ounce plastic cups (paper cups tend to break at
     the seam as the contents freeze) Morton brand table salt Morton
     brand rock salt 1/2 cup measure Stop Watch

     Procedure

     To answer the questions posed above, a balanced 2 x 4 factorial
     design was employed with amount of salt identified as a factor
     consisting of four levels (i.e. no salt, 1/2 tsp, 1 tsp, 1 tbsp),
     and the other factor being type of salt with two levels (i.e.
     table salt, rock salt).  Three replications were conducted within
     each cell for a total of 24 runs.  A p-level of .05 was identified
     for statistical significance prior to the data collection phase of
     the project.

     Twenty-four small plastic cups were each labeled with a number
     designating type of salt, and a letter A-D indicating amount of
     salt. Each plastic cup was then filled with 4 ounces of tap water
     and placed in the freezer overnight (approximately 16 hours).

     Since salt could not be emptied into all of the ice cups
     simultaneously, the remaining 18 plastic cups were each labeled
     and then used to hold an amount and type of salt corresponding to
     one of the experimental conditions.  After the ice cups had been
     removed from the freezer, each salt cup was quickly emptied into a
     corresponding ice cup with matching identification so as to
     minimize the time interval between the application of salt to the
     first and last cups.

     After the last cup of salt had been emptied into the appropriate
     ice cup, the stopwatch was started.  Room temperature during the
     data collection phase was approximately 72 degrees Fahrenheit. 
     The time was recorded for each cup when ice was no longer visible
     in that cup.

_S_o_u_r_c_e:

     Taken from a 1999 project by Wayde D. Johnson

